1. Field of the Invention
The present invention relates to a liquid ejection head and a liquid ejection method of performing printing by ejecting a liquid, and more particularly to a method of joining a plurality of liquid droplets during ejection.
2. Description of the Related Art
A print head, used in ink jet printing and that performs printing by ejecting a liquid onto a print medium, applies energy such as heat to the liquid to cause a status change in the liquid accompanied by a rapid liquid volume change, thereby ejecting the liquid from ejection openings by a status change-produced force.
With this ink jet printing system, high-quality images can be printed at high speed with low noise. Further, the ink jet printing system is able to arrange liquid ejection openings at high density in the print head. The ability of the ink jet printing apparatus to arrange the ejection openings at high density provides many advantages. Among others, the printing apparatus itself can be reduced in size and color images obtained easily. Because of these advantages, the ink jet printing method in recent years has found an increasingly wide range of use with office equipment, such as printers, copying machines, and facsimiles, and also in industrial systems such as cloth pattern printing apparatuses.
In such an ink jet printing system, a liquid to be ejected gets elongated before being disconnected from the body of liquid to form a droplet that lands on a print medium. At this time, the liquid droplet intended to reach the print medium has a front end part of the droplet (main droplet) and a column part (ink tail). Generally, the ink tail is smaller in volume and slower than the main droplet and thus lands on the print medium at a position deviated from that of the main droplet, degrading the print quality. It is therefore necessary to disconnect the ink as early as possible. To meet this requirement, it is desired that the ink droplet ejected from the ejection opening be as small in total volume as possible. This is because a reduced volume of liquid droplet naturally results in an early disconnection. That is, one droplet is divided into a plurality of smaller droplets to reduce the volume per droplet as they are ejected.
One example method based on this idea involves ejecting a plurality of droplets from a plurality of ejection openings in a manner that joins them together on the fly. By ejecting the liquid in the form of a plurality of small droplets, they can be split from the body of the liquid early. Combining the small droplets into a larger droplet on the fly can reduce the influence of air flow, preventing a possible degradation of print quality.
Japanese Patent Laid-Open No. 06-286138 describes an example method of ejecting small liquid droplets and then joining them into a larger droplet. With this method, two ejection openings are provided for one ink flow path, and two small ink droplets ejected from the two ejection openings are combined to form a larger droplet on the fly.
The smaller volume of droplet, however, has a disadvantage in that it is more easily affected by air resistance and therefore air flows around the print head. This will result in positional deviations of printed dots on the print medium, degrading the print quality. It is therefore desired that an ejected ink droplet be small in volume as it leaves the nozzle but, after it has parted from the nozzle, become larger on the fly. Therefore, the construction of Japanese Patent Laid-Open No. 06-286138 has no problem when two droplets fly under an ideal condition. But in practice, an ejection state of individual ink droplets sometimes varies according to actual conditions of use. The ejection state variations (deflections of ejection direction and variations in ejection volume) may result in a combined ink droplet being deflected from an intended direction and, in a worst case, small ink droplets failing to join together.
A distance between two holes or ejection openings, that causes two independent ink droplets to have a columnar shape as they leave the ejection openings and then to combine together on the fly to finally land on a print medium as a single droplet, is very subtle. So, it is difficult, with the present construction as is, to eject ink droplets in a way that can stably keep their ejected state. Even if the ejection of independent droplets and the subsequent joining of droplets should be able to be realized under a certain condition, since the two-hole distance is based on the subtle condition, any change in conditions during use, such as an ink property and a surface state of ejection openings, can result in the independent droplets failing to combine or the ink being ejected as a single dot from the beginning, thus degrading the print quality.